The present invention relates generally to gas turbine engines, and, more specifically, to the manufacture of airfoils therein.
A turbofan aircraft engine is one form of a gas turbine engine which includes in serial flow communication a fan, multistage axial compressor, combustor, high pressure turbine (HPT), and low pressure turbine (LPT). During operation, air is channeled past the fan and a portion thereof enters the compressor for being pressurized and then mixed with fuel in the combustor and ignited for generating hot combustion gases. The combustion gases flow in turn through the HPT and LPT which extract energy therefrom for powering the compressor and fan, respectively.
The engine includes various airfoils in the form of rotor blades such as those found in the fan, compressor, and turbines. Additional airfoils are in the form of stator vanes also found in the compressor and turbines.
Since the turbine blades and vanes are subject to the hot combustion gases during operation, they are typically hollow for channeling therethrough a portion of air bled from the compressor for cooling. The high pressure turbine vanes and blades experience the most severe temperature environment and therefore require corresponding cooling thereof for obtaining a suitable useful life during operation.
Typical cooling features found in turbine blades include multi-pass serpentine cooling passages; turbulators in various forms in the cooling passages for enhancing cooling effectiveness; various holes through the airfoils for preferentially cooling the pressure and suction sides, leading and trailing edges, and tip both convectively or by generating cooling films; and impingement baffles inside the airfoils having impingement holes which direct a portion of the cooling air in impinging jets against the inner surface of the airfoils for enhancing internal cooling thereof.
Since these airfoils are initially internally cooled, the internal cooling features must be suitably formed therein. This is typically accomplished by casting the airfoils to near-net shape both internally and externally. Externally the airfoil may then be machined to final or finish configuration to meet the close tolerances required for maximizing engine performance. However, it is not possible to machine the internal cooling features of the airfoils, which may be formed only to the accuracy provided by the specific casting method.
Furthermore, the ability to use impingement baffles in turbine vanes and blades is limited by the ability to assemble the baffles therein since it is not practical to separately cast perforated baffles therein. The individual baffles are preformed and inserted into the airfoils from either of their two opposite ends as practical, and then fixedly bonded at one end thereof inside the airfoil for allowing unrestrained differential thermal expansion and contraction relative to the airfoil itself.
Gas turbine engine performance is primarily affected by the temperature of the combustion gases, with efficiency increasing as temperature increases subject to the high temperature strength of the materials being used and the available cooling thereof. Since the internal cooling features of typical turbine vanes and blades are limited in accuracy and detail by the particular casting methods utilized, further advances in engine efficiency may be obtained by further advances in the internal airfoil cooling features.
Turbine airfoils may also be manufactured in two parts so that the internal cooling features thereof may be preformed prior to assembly of the two parts. In view of the hostile environment of the gas turbine airfoils, the two parts must then be adequately joined together for maintaining the structural integrity thereof. This may be accomplished by the conventionally known process of diffusion bonding, which is a solid state brazing or welding process which joins together the parts at the interface thereof.
However, diffusion bonding requires precise mating surfaces without unacceptably large surface irregularities therebetween which would degrade diffusion bonding thereat. Correspondingly, machining of the mating surfaces requires precise tolerances which is difficult and expensive to achieve, especially over a 3-D interface matching the twist of the airfoil. Accordingly, the cost of a diffusion bonded airfoil may approach the cost of manufacturing two conventionally cast single blades, and is therefore prohibitively expensive.
Accordingly, an improved method of making hollow gas turbine engine vanes and blades is desired for further increasing the efficiency of operation of the engine.